Prior heat exchangers have included a plurality of round or oval tubes having a smooth or seamless surface that are typically formed by welding These welded tubes have an unconstricted flow passage and are attached to a pair of headers to form a heat exchanger assembly. The tubes are joined to the headers by either vacuum brazing or controlled atmosphere brazing ("CAB"). Vacuum brazing and CAB are well known in the art.
Vacuum brazing is furnace brazing in a vacuum that eliminates the need for any flux. In operation, the assembly is heated in a furnace up to brazing temperature which takes about an average of 15 minutes. The assembly is then held at brazing temperature for about 1 minute and then quenched or air-cooled as necessary. Controlled atmosphere brazing ("CAB") is widely used for the production of high quality joints. CAB is not intended to perform the primary cleaning operation for the removal of oxides or other foreign materials from the parts to be brazed. Accordingly, fluxes are used with a controlled atmosphere to prevent the formation of oxides and to break up the oxide surface to make the surface more wettable.
These brazing techniques form a sufficiently strong bond between the headers and the prior round or oval tubes. Recently, folded-type or seamed tubes have been developed for use in heat exchangers. These tubes have a constricted flow passage. When the above described brazing techniques are applied to folded-type or seamed heat exchanger tubes, they yield a weak tube-to-header joint that can result in leakage of heat exchanger fluid or other failure of the heat exchanger apparatus under the combined influence of heat, vibration, and pulsating pressure. The primary cause of the weak tube-to-header joints is a poor fillet at the tube-to-header joint. Additionally, a poor fillet also occurs between the folded seam and inner surface of the tube. If the bond is weak at either of these locations, leakage of heat exchange fluid from the tubes results. The bond must also be strong if the heat exchangers are used in automobiles to withstand high vibrations, high temperatures, and long periods of use.
Various corrective techniques have been attempted to provide a better fillet at the tube-to-header joint and between the tube fold and tube inner surface. For example, elevating the brazing temperatures and increasing the brazing cycle times were two attempted techniques. However, these techniques removed even more cladded filler (fillet) from the surface of the headers, resulting in an even weaker tube-to-header bond.
Other corrective techniques included increasing the amount of clad on the outside of the folded-type tubes or using clad on the inside of the folded-type tubes. These techniques did not provide any appreciable increase in strength between the tube-to-header joint or tube fold to inner surface joint and only resulted in wasting the excess clad added to the tubes, resulting in increased cost. Another technique included utilizing cladded fins in the heat exchanger. However, this also increased the cost without providing any appreciable change in the strength of the tube-to-header joint or tube fold to inner surface joint. Another attempt to increase the bond at the tube-to-header joint and the tube fold to inner surface joint was to resize the tubes after assembly was completed. However, this also failed to provide any appreciable increase in strength of the tube-to-header joint or tube fold to inner surface joint. Thus, there has been no successful way to incorporate folded-type tubes into a heat exchanger assembly with a strong fillet at the tube-to-header joint or at the tube fold to inner surface joints.